a) Field of the invention
The present invention relates to a lens system which adopts a graded refractive index lens component and is to be used in cameras, etc.
b) Description of the prior art
Examples of photographic lens systems having F numbers not exceeding 2 or having large apertures which are to be used in silver salt cameras, Gaussian type lens systems are known. A lens system of this type comprises as many as six or seven lens elements, requires a high manufacturing cost and is large in dimensions thereof.
Further, known triplet type lens systems are known as lens systems each comprising a small number of lens elements. However, lens systems of the triplet type have a common defect that they have F numbers as large as 2.8 or higher, small apertures and produce remarkable curvature of field.
In order to correct the defects of the lens systems of the types described above, it is conceivable to use aspherical surfaces in these lens systems. However, it is impossible to remarkably reduce the number of the lens elements of the Gaussian type of lens systems by using aspherical surfaces therein, because Petzval's sums and chromatic aberration cannot be corrected by using the aspherical surfaces though spherical aberration, coma, distortion, etc. can be corrected favorably by using the aspherical surfaces. Further, it is impossible to correct curvature of field by using aspherical surfaces in the triplet type of lens systems.
Recently, it has been conceived to use, in lens systems, graded refractive index lens elements having refractive indices varying from location to location thereof. In particular, use of radial type graded refractive index lens elements makes it possible to correct spherical aberration, as favorably as in cases where it is corrected by using aspherical surfaces, in addition to Petzval's sums and chromatic aberration.
A lens system using radial type graded refractive index lens elements is exemplified by the lens system described on Pages 993-998, Applied Optics, Vol 21. This lens system has a composition illustrated in FIG. 1, wherein radial type graded refractive index lens elements which have shapes of concave lens elements and are made of materials having positive refractive powers are disposed symmetrically with regard to an aperture stop. This lens system consists of lens elements in a number as small as 1/3 of the number of the homogenous lens elements required for composing an ordinary lens system and has aberrations favorably corrected.
Further, as another example of a lens system using graded refractive index lens elements, there is known the lens system disclosed by Japanese Patent Kokai Publication No. Sho 61-4,012. This lens system is composed of two or three lens elements in total: a radial type graded refractive index lens element and one or two homogenous lens component.
The lens system described on pages 993-998 of Applied Optics Vol. 21 requires a high manufacturing cost since the two radial type graded refractive index lens elements are expensive. Further, when radial type graded refractive index lens elements are actually manufactured, these lens elements have refractive index distributions different from design refractive index distributions, whereby it is necessary to correct the differences by slightly varying curvature on the lens surfaces from design curvature at the manufacturing stage. When a lens system is composed of two radial type graded refractive index lens elements which produce differences in refractive index distributions as described above, however, it is impossible to correct the differences simply by varying curvature on the four lens surfaces. In such a case, the graded refractive index lens elements must be manufactured so as to have refractive index distributions which strictly coincide with design distributions and can hardly be prepared in practice.
Furthermore, the lens system disclosed by Japanese Patent Kokai Publication No. Sho 61-4,012 cannot have a large aperture due to a fact that it produces remarkable coma.